The present invention relates to a device for use in conjunction with continuous welding machines, by means of which to pre-calibrate hollow cylindrical bodies in readiness for seam welding.
The prime requirement in such machines is that of positioning the rolled sheet of material during its progress toward the weld station, in such a way as to obtain a shape as near cylindrical as possible, and a feed as continuous and regular as possible. Such a requirement is of fundamental importance in welding machines of the type in question, as even a slight variation in the position of the two edges of the sheet lapped for joining can leas to a defective weldment, as also can an irregular feed action.
In the particular instance of seam welding machines for hollow cylindrical objects, and in particular tin cans, the sheet of metal destined to form the cylindrical wall of the can is fed toward the weld along a horizontal arm. During its passage along the arm, the rolled-up sheet is directed first through a pre-calibration device, and thereafter through a final calibration device at which contact is made with two electrode rollers, positioned internally and externally of the hollow body, respectively; it is these rollers that effect the weld.
The pre-calibration device comprises a plurality of sets of profiled rollers or roller stations, ranged around successive parallel circumferences. This device is followed, along the feed direction, by a further set of rollers constituting the calibration device proper, which consist in calibrated wheels describing an internal circumference of diameter identical to that of the size of a can; the rollers of the pre-calibration station are shaped in such a way as to match the curvature of the hollow body, and carried in spring-loaded clevis-type supports anchored to the main structure of the machine. The prior art also teaches the use of a Z-section guide positioned longitudinally in relation to the can, above the pre-calibration and calibration devices, which affords opposed profiles converging onto the weld, against which the lapped edges of the sheet of metal locate during pre-calibration. Thus, on arrival at the weld rollers, the can body is completely encompassed and held firmly against the profiles of the `Z` by the pressure of the pre-calibration rollers, which is applied radially and counteracts the tendency of the rolled sheet to spread.
One of the limitations of such a system consists precisely in the structural embodiment outlined, and more exactly, in the spring-loaded connection between the single roller and the machine frame. The springs are required to withstand dynamic distortions occurring in a particularly short space of time, given the fast tempo at which welding is performed, and thus operate in notably critical conditions; moreover, the device as a whole is complicated and costly.
Furthermore, can bodies which exhibit dimensional defects may jam on encountering the spring-loaded rollers at high speed, with the result that the leading edge can deform on entering between the electrodes. Another serious drawback stems from the risk of jamming to which the spring rollers are subject; this is caused by dust (tin, in most cases) that falls on the clevis supports and will accumulate unless frequently cleaned. In addition, the spring rollers require a complex and laborious adjustment procedure in order to select the radial pressure transmitted to the can.
In view of the foregoing, the object of the present invention is to embody a pre-calibration device that will permit of guiding can bodies for welding and controlling their dimensions, even at high rates of production (e.g. 60 meters/min and more), and which is simpler to construct and control and offers greater dependability than prior art types.